This invention relates to a pneumatic servomotor used for assisting in the braking of a motor vehicle.
More particularly, the present invention relates to a pneumatic servomotor for an assisted braking of a motor vehicle, of the type comprising a rigid casing, in which a transverse partition wall is movable, thus defining in an airtight manner a front chamber under a first engine negative pressure, and a rear chamber under a second pressure, varying between the engine negative pressure and the atmospheric pressure; of the type including a moving piston, integral with the moving partition wall and capable of biasing a rear end of an actuating rod for a brake master cylinder, said actuating rod being slidably arranged in relation to the moving piston, against a first return spring mounted inside the front chamber; and of the type including a control rod, moving inside the piston, in a selective manner as a function of an axial input force, exerted in the forward direction against a return force applied to the rod by a second return spring; of the type according to which the control rod is biased towards either an intermediate actuation position or an end actuation position, resulting from the applying of the input force at a determined high speed; of the type comprising a plunger, arranged at the front part of the control rod inside the piston, and a three-way valve, including at least an annular seat borne by a rear section of the plunger, and capable of varying the second pressure, prevailing within the rear chamber, particularly by connecting the front chamber with the rear chamber when the control rod is in the rest position, or by gradually connecting the rear chamber with the atmospheric pressure when the control rod is actuated; and of the type in which, in the end actuation position of the control rod, a finger-forming front end of the plunger biases a reaction disk integral with the rear end of the actuating rod, so as to transmit the reaction force of the actuating rod to the plunger and to the control rod.
Numerous examples of such pneumatic servomotors are known, for instance as disclosed in documents U.S. Pat. No. 3,470,697, FR-A-2 532 084 and FR-A-2 658 466.
It has been established that quite a number of drivers, when confronted with a braking situation requiring the jamming on of the brakes, would suddenly apply the brakes but afterwards, underestimating the risks actually incurred, would somewhat release the braking force at the very time when a high braking force should have been maintained in order to avoid an accident.
As a matter of fact, in the case of a full-braking situation, in which a maximum braking force is applied on the control rod, the actuation of the control rod causes the finger-forming plunger to be actuated, which fact results in the maximum opening of the three-way valve and, therefore, the rear chamber is subjected to the atmospheric pressure. Thus, the moving partition wall travels forward and the end of the plunger contacts the reaction disk, integral with the rear face of the moving piston.
Therefore, the force, which is applied onto the moving piston when the control rod reaches the end of its stroke, results from the assisting force, arising from the pressure difference between each side of the moving partition wall, and from the force exerted by the finger-forming plunger onto said moving piston. Besides, the driver feels the braking reaction force, which is transmitted from the moving piston to the plunger, through the reaction disk.
Now, in the case of a full-braking situation, accompanied by the swift travel of the control rod, the plunger may touch the reaction disk and therefore give the driver the feeling of a maximum braking action even before the pressure difference between the front and rear chambers actually reaches its maximum value, which may lead the driver to release the braking force even though it should be maintained so as to profit by the maximum braking force.
In order to cope with said difficulty, it is a well-known practice to provide a pneumatic servomotor comprising a finger, which is slidably mounted in relation to the plunger, and a unidirectional clutch device permitting, when the braking force is applied at a determined speed, the indirect locking of the actuating rod, through the locking of the finger relative to the moving piston, independently of the plunger position.
Such a servomotor is described, more particularly, in document WO-A-99/59 854. This document discloses and illustrates a servomotor, comprising a finger, slidably fitted at the front end of the plunger and capable of being locked in relation to the moving piston by means of a detent controlled by a key, which is displaceable between a rest position, in which it bears on the casing of the servomotor, and a rocked position, in which it no longer bears on the casing of the servomotor but in which it actuates the detent, cooperating with a flange of the finger.
A servomotor of the previously-described type makes it possible to cope with said difficulty, in that the plunger is locked into contact with the reaction disk by means of a sleeve, as a consequence of which a maximum force is maintained on the rear face of the moving piston, even though the driver may have released the braking force in part.
Other devices are known from the state of the art but all of them have the same disadvantage, unlike a servomotor without a unidirectional clutch device, of requiring, as it is manufactured, the separate machining of a sleeve and of a finger. Such design means a considerable increase in the manufacturing cost of said servomotor.
Besides, the locking of the actuating rod can be achieved in an indirect manner only, by the locking of the finger, i.e. through the reaction disk, made of an elastomeric material. Therefore, the accurate locked position of the actuating rod depends on the volume variations of the reaction disk, owing to the reaction force applied to the actuating rod by the brake master cylinder, which it controls.
In order to cope with such a disadvantage, the present invention provides a novel design of a servomotor, comprising a unidirectional clutch device including a finger, integral with the plunger, and in which the locking of the actuating rod relative to the moving piston is directly achieved, i.e. independently of the reaction disk.
Therefore, it is the object of the present invention to provide a servomotor of the above-described type, characterised in that it comprises a unidirectional clutch device which, when the input force is applied at the determined speed, is capable of taking up an active position, in which it locks the actuating rod in relation to the moving piston in an end front axial position, in which the reaction force is transmitted to the moving piston, independently of the reaction disk and independently of the position of the plunger and of that of the control rod.
According to other features of this invention:
the unidirectional clutch device comprises expandable means for an axial insertion, in the active position, between the moving piston and the rear end of the actuating rod, so as to lock the latter into its end front axial position;
the plunger has a finger-forming front free end section, which is slidably fitted within a complementary front bore of the piston;
the front bore of the piston opens into a cup, integral with the rear end of the actuating rod, said cup being slidably mounted on a cylindrical bearing surface of the piston, and receiving the reaction disk;
a rear pawl of the actuating rod extends axially rearwards from the bottom rear face of the cup, passes through the reaction disk and is accommodated, with an axial clearance, within a blind axial hole provided in the plunger, so as to let the plunger, when the input force is exerted at a determined high speed, bias directly the actuating rod towards its front end position;
the expandable means of the unidirectional clutch device comprise several balls at regular angular spacing about the cylindrical bearing surface of the moving piston, and fitted so as to be movable between a radial position away from the axis of the moving piston, and corresponding to an inactive position of the unidirectional clutch device, and a radial position, which is closer to the axis of the moving piston and which corresponds to the active position of the unidirectional clutch device, and in which they are capable of coming axially between a rear end face of the cup and a front face of the moving piston, so as to lock the actuating rod in its end front axial position in relation to the moving piston;
the rear pawl of the actuating rod is mounted with a given clearance relative to a bottom of the blind hole provided in the plunger, in such a way that, when the input force is exerted at a determined speed, the plunger pushes the cup in the forward direction by a determined distance, for the balls to come axially into contact with the cylindrical bearing surface of the moving piston, between the rear end face of the cup and the front face of the moving piston;
the rear end face of the cup exhibits a bevel with the purpose of:
providing a support for the balls, in the inactive position of the unidirectional clutch device; and
guiding the balls into contact with the cylindrical bearing surface of the moving piston, in the active position of the unidirectional clutch device;
the expandable means comprise a tubular sleeve, slidably fitted about the cylindrical bearing surface, in a coaxial bore provided in the moving piston, said sleeve being resiliently biased rearwards and having a rear inner bevel, substantially at right angles with the balls and the purpose of which consists in:
maintaining the balls in their remote radial position, resting on the bevel-forming rear end face of the cup, and on a supporting washer contacting the front face of the moving piston, in the inactive position of the unidirectional clutch device;and
biasing the balls towards their closer radial position, so that they may come between the bevel-forming rear end face of the cup and the washer, into contact with the cylindrical bearing surface of the piston, in the active position of the unidirectional clutch device;
the expandable means Of the unidirectional clutch device comprise at least one ring, made of an elastomeric material and glued about the cylindrical bearing surface of the moving piston and about the cup, said ring being deformable between an expanded radial position, corresponding to an inactive position of the unidirectional clutch device, and a compressed radial position, which corresponds to the active position of the unidirectional clutch device and in which it can lock the cup in an advanced position, so as to put the actuating rod in its end front axial position in relation to the moving piston;
the rear pawl of the actuating rod is mounted with a determined clearance relative to a bottom of the blind hole made in the plunger so that, when the input force is applied at a determined high speed, the plunger pushes the cup in the forward direction by a determined distance, with the result that a given space is made available for an immediate occupying by the elastomeric ring, the latter being radially biased into compression by a rear inner bevel in a tubular sleeve, slidably mounted and resiliently returned about the cylindrical bearing surface of the moving piston, in a coaxial bore provided in the moving piston;
the unidirectional clutch device comprises means for the forward driving of the tubular sleeve, against its return means, when the input force is released so as to release the unidirectional clutch device as well; and
the moving piston includes a substantially radially directed cavity which receives a key, slidingly fitted on the plunger and comprising two opposite branches protruding out of the cavity and capable of abutting against the respective walls of the casing, when the control rod is in the rest position, with the result that a front face of one of the branches biases a slide forwards, said slide being slidingly fitted inside a bore provided in the moving piston and opening facing a rear edge of the sleeve in such a manner that, when the input force is released, the tubular sleeve is driven forwards again, against its return means.
Other features and advantages of the present invention will be apparent from the following detailed description, when taken in conjunction with the accompanying drawings.